Are you using HDMI straight connection from the receiver to the Intel® NUC?
I recommend using high quality HDMI cables.
As you mention the Intel® NUC supports DTS, Dolby DTS and Dolby HD 7.1 and 5.1 as you can see at:
Please see page 23.
Also, I found a link that can be helpful for you, you can see it here:
Please confirm the current configuration at the Windows* 7 "Volume Control Options". and the current "Playback Devices" by right clicking on the Volume icon located on the bottom right of the screen.
Also please make sure all drivers are installed and up to date. You could use Intel® Driver Update Utility
It's been a while from the time I asked the question. I upgraded to Window 10 since than and learnt something in the process. The truth is playing videos from internet (like YouTube) in a browser not necessarily supports sound formats like Dolby Digital or DTS. Even Netflix and Amazon Video players don't support quality audio via browser. Microsoft Edge browser and Windows 10 started to support those recently. I use Netflix app for Windows 10. Works good. To play digital videos from a hard drive requires video player software. I installed Plex and VLC and enjoying sound and video.